For instance, an endless metal belt to be used in a belt-type continuously variable transmission mounted in a vehicle and used to transmit drive power is composed of a plurality of endless metal rings (e.g., nine rings) stacked or laminated in close contact with each other, the endless metal rings each having a circular-arc cross section and different circumferential lengths. In general, as a material of an endless metal ring, there is so far known maraging steel having superior strength characteristics. Further, an endless metal ring is made of a steel-strip annular member that has been subjected to rolling and thereafter to a solution treatment, a circumferential length adjusting (calibration), an aging-nitriding treatment in this order to enhance fatigue strength (for example, see Patent Document 1).